There are a lot of factors to be taken into account when you are developing a product that is comfortable for the audience to play. It all starts with a well-structured UI with the minimum steps to get a result and well-designed control elements. A good interface is "invisible", a player simply does not notice that there is an intermediary between him and a game, because it all works so smoothly; hence, there is no reason to pay attention to it. To develop such a system, you need to think of every detail by following some basic rules. Always keep in mind some bad examples and try to avoid them in your own project.
There are some people that need some special care and many of them are devoted gamers. It won't take much time or effort to make a game suitable for their needs. To prepare a product which is friendly for color-blind people, remember that only some important indicators and color schemes must be reinvented. Some controversial color pairs such as red/green should be avoided...